crypto: sig - Remove some unused functions

These functions are defined in the sig.c file, but not called elsewhere,
so delete these unused functions.

crypto/sig.c:24:34: warning: unused function '__crypto_sig_tfm'.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Closes: https://bugzilla.openanolis.cn/show_bug.cgi?id=5701


Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>